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500 000-Year records of carbonate, organic carbon, and foraminiferal sea-surface temperature from the southeastern South China Sea (near Palawan Island)

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500 000-Year records of carbonate, organic carbon,

and foraminiferal sea-surface temperature from the

southeastern South China Sea (near Palawan Island)

Min-Te Chen

a;

, Liang-Jian Shiau

a

, Pai-Sen Yu

a

, Tzu-Chien Chiu

b

,

Yue-Gau Chen

b

, Kuo-Yen Wei

b

a Institute of Applied Geophysics, National Taiwan Ocean University, Keelung 20224, Taiwan R.O.C. b Department of Geosciences, National Taiwan University, Taipei, Taiwan R.O.C.

Received 22 August 2001; received in revised form 29 April 2002; accepted 1 February 2003

Abstract

High-resolution records of planktic foraminifer sea-surface temperature (SST) and biogenic sediment components of carbonate and total organic carbon (TOC) concentrations were determined in an IMAGES giant piston core spanning Vthe last 500 000 years, taken near the western slope of Palawan Island in the southeastern South China Sea (SCS). The records provide information of paleoceanographic and paleoclimatological variations linked to East Asian monsoon systems in the SCS, the largest marginal sea of the western Pacific. Constrained by planktic foraminifer (Globigerinoides ruber) oxygen isotope stratigraphies, the records show a lowering of faunal SSTby V3‡C during glacial stages, indicating significant cooling in the glacial western Pacific climate. In general, they show low-frequency patterns with high SSTs, high carbonate content, and low TOC content during interglacial periods, and exhibit low SSTs, low carbonate content, and high TOC content during glacial periods. The carbonate content variations indicate that the sediment composition is mostly controlled by terrigenous inputs, which are associated with sea-level fluctuations in the SCS during past glacial^interglacial stages. The low SST and high TOC content indicate cooling and high productivity conditions in the surface oceans of the SCS, which also reflect a condition of intensified winter monsoon winds associated with glacial boundary conditions. Some rapid, high-frequency oscillations of the SSTand TOC found in the records are coincident with intervals of intensified winter or summer monsoons from the Arabian Sea, implying that the Asian monsoon systems had wider regional effects than previously assumed. Time-series analyses reveal that variations in the SST, carbonate and TOC contents of this record contain statistically significant concentrations of variance at orbital frequency bands, namely 100 kyr31, 41 kyr31, and 23 kyr31, suggesting that both ice volume and orbital solar insolation changes are potential mechanisms for the SCS monsoon variations.

; 2003 Elsevier Science B.V. All rights reserved.

Keywords: planktic foraminifer; East Asian monsoon; South China Sea

1. Introduction

The strong winds associated with East Asian

* Corresponding author. Tel. : +886-2-2462-2192 x6503; Fax: +886-2-2462-5038.

E-mail address:mtchen@mail.ntou.edu.tw(M.-T. Chen).

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monsoons play an important role in governing seasonal patterns in the surface ocean hydrogra-phies of the South China Sea (SCS). The mon-soon, driven by the atmospheric pressure di¡er-ence between land and ocean, generates seasonally reversible current systems in response to prevail-ing wind direction (Shaw and Chao, 1994). Dur-ing the winter months (November^April), north-easterly winds bring cold air from the East Asian continent as well as from Chinese coastal water into the SCS through the Taiwan Straits, leading to the development of strong latitudinal gradients in sea-surface temperature (SST). This system is replaced during the summer months (May^Octo-ber) by southwesterly winds, which bring an in-£ux of warm and humid surface air. Warm Indian Ocean surface water also £ows northward over the Sunda Shelf into the southern SCS. The summer season SSTs are high (V28‡C) with rel-atively insigni¢cant spatial variation. Observatio-nal and modeling studies indicate that the prevail-ing monsoon wind-induced Ekman pumpprevail-ing in the surface oceans results in winter upwelling o¡ the northwestern Philippines (Shaw et al., 1996) and in summer upwelling along the eastern Viet-nam margins (Wiesner et al., 1996 ; Ho et al., 2000).

Paleoceanographic investigations using deep-sea records of the past few glacial^interglacial cycles suggest that surface ocean temperature var-iability in the SCS is either dominated by mon-soon wind processes (Huang et al., 1997a,b ; Chen and Huang, 1998; Wang et al., 1999a,b ; Chen et al., 1999) or largely controlled by sea-level £uctu-ations (Wang and Wang, 1990; Wang et al., 1995 ; Pelejero et al., 1999a,b). During the last glacial maximum (LGM), relatively cold surface temper-atures over the East Asian continent and rela-tively warm western Paci¢c Ocean temperatures may have driven strong northeasterly winds, which in turn induced strong mixing and prob-ably high productivity in the surface ocean of the SCS. The strong winds may have deepened the mixed-layer depth, and also carried large amounts of dust that may have been incorporated into biogenic particle aggregates, thereby increas-ing the transfer rate of organic matter from the surface to the deep sea (Ittekkot et al., 1992). The

transfer rate may have been even higher, since large quantities of riverine nutrients and mineral particles are laterally advected to the SCS from large rivers such as the Changjiang (Yangtze Riv-er) and Zhunjiang (Pearl RivRiv-er) (Ittekkot et al., 1991 ; Wiesner et al., 1996). On the other hand, lowering sea-levels gave rise to an emerged con-tinental shelf around the SCS, and consequently restricted the in£ow of warm waters from the open oceans. For example, the emergent Sunda Shelf in the southern part of the SCS may have precluded any exchange with tropical Indian and Paci¢c surface waters. These reduced in£ows may have led to decreased SSTs in the SCS. Under glacial low sea-level conditions, the depocenter may have shifted toward the outer shelf and con-tinental slope (Schonfeld and Kudrass, 1993). This process may have brought more suspended ¢ne-grained £uvial terrigenous sediments to the slope with an increased accumulation rate, a con-dition favorable for enhanced organic matter preservation in marine sediments (Mu«ller and Suess, 1979; Sarnthein et al., 1988).

Previous SCS paleoceanographic studies con-centrated on the past glacial^interglacial varia-tions. There is a lack of long records (V500 000 years) from the SCS to constrain the past varia-tions at orbital time scales. Moreover, there is also a lack of comparisons on paleomonsoon re-cords of the SCS and of the Arabian Sea, Indian Ocean. To this end, here we examined variations in planktic foraminifer faunal assemblages and stable isotopes, as well as biogenic carbonate and organic carbon concentrations (wt%) and mass accumulation rates (MARs) based on an IMAGES (International Marine Global Change Study) core (MD972142) from the southeastern SCS near Palawan Island (Fig. 1). The modern SSTat the coring site is V28‡C and the seasonal variation of the SSTis minimal (NOAA, 1994). This site is located far away from the large rivers that drain into the SCS. Biogenic sediment pres-ervation resulting from lateral advection of river-ine material supply is thought to be minimal in the SCS, therefore the core taken from this area is ideal for examining past surface ocean variability associated with East Asian monsoons in the SCS. In this study, we present foraminifer

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SSTesti-mates and biogenic sediment productivity proxies from this record, spanning Vthe past 500 000 years. The speci¢c objectives of this study were to : (1) document variations in planktic foramini-fer faunal SSTs and productivity-related biogenic sediment component concentrations and MARs in the SCS over the past 500 000 years ; (2) deter-mine the timing and magnitude of variations in the fauna SSTand biogenic sediment productivity estimates associated with glacial^interglacial changes ; (3) identify possible regional climatic processes (for example, East Asian monsoons) which may have a¡ected both the SSTand pro-ductivity changes in the SCS ; and (4) compare paleomonsoon records of the SCS and the Ara-bian Sea, Indian Ocean.

2. Data and methods

For the purposes of this study, an IMAGES core (MD972142 (12‡41.133PN, 119‡27.90PE; water depth 1557 m)) was obtained from the west-ern continental slope of Palawan Island, in the southwestern SCS (Fig. 1) by the French research vessel Marion Dufresne on its 1997 IMAGES III cruise (Chen et al., 1998). The sediments in this core are composed of light olive gray muds to foraminifer nannofossil oozes, interrupted by 18 visually distinguishable thin tephra layers. The thickness of these tephra layers ranges from 0.5 to 10 cm, with most s 1 cm. The water depth for taking this core was well above the regional lyso-cline (V3500 m, Thunell et al., 1992), which

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sures good carbonate preservation at this site. The core was sampled every 4^12 cm and a total of 300 samples were analyzed for planktic foramini-fer faunal assemblages, 353 samples for planktic foraminifer oxygen and stable carbon isotopes (Globigerinoides ruber), and 560 samples for bio-genic carbonate and organic carbon concentration analysis. The intervals containing tephra layers were not used.

The 353 stable isotope measurements of the planktic foraminifer Globigerinoides ruber (white morphotype with size range 250^300 Wm) were made at National Taiwan University using a Fin-nigan Delta Plus mass spectrometer with a Kiel automated carbonate device. Analyses were dupli-cated for every sample with a precision of 0.08x PDB. Average values from the screened dupli-cated results were then calculated (Fig. 2). Con-struction of the stratigraphy of core MD972142 was mainly based on a comparison of the planktic foraminifer N18O data from this core with a

low-latitude N18O stack (Bassinot et al., 1994) (Fig. 2)

and on the last appearance datum of G. ruber (pink morphotype) of V120 000 years ago (Thompson et al., 1979) based on biostratigraphic

examination on this core. The planktic foramini-fer faunal census data were processed using stan-dard techniques that have been applied in many previous SCS sediment studies (Chen et al., 1998,

1999 ; Chen and Huang, 1998) conducted at the

Laboratory of Paleoceanography, National Tai-wan Ocean University. The planktic foraminifera were picked from splits of the s 150-Wm size frac-tion containing approximate 300 specimens. A to-tal of 27 faunal species were identi¢ed and the relative abundance of each species is expressed as a percentage of the total faunal assemblage (Fig. 3). The dominant species identi¢ed in this core were : G. ruber, Neogloboquadrina dutertrei (+Neogloboquadrina pachyderma (right coiling)), Globigerinoides sacculifer, Globigerina calida, Pul-leniatina obliquiloculata and Globigerinita gluti-nata.

We estimated the SSTby using a planktic for-aminifer transfer function method. This statistical approach utilizes regression equations that relate modern coretop faunal distributions to overlying surface water temperatures (Imbrie and Kipp, 1971). In the western Paci¢c, SSTestimates have historically been based on an old-version linear

Fig. 2. The oxygen isotope (Globigerinoides ruber) stratigraphies of the IMAGES core MD972142 which is estimated to represent early stage 13 through the Holocene. The age control points are determined by comparing the curve of MD972142 N18O and a

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Fig. 3. Relative abundances of six dominant species of planktic foraminifera for core MD972142, plotted against age and com-pared to downcore N18O stratigraphy. Shaded intervals indicate glacial periods.

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transfer function FP-12E (Thompson, 1981). This set of transfer functions has previously been ap-plied in a project that estimated the LGM SST patterns of the western Paci¢c (CLIMAP, 1981). A methodologically di¡erent technique, SIM-MAX-28 (P£aumann et al., 1996), has been tested recently for estimating SCS SSTrecords ( P£au-mann and Jian, 1999 ; Jian et al., 2001). Since such paleo-SSTestimates in the western Paci¢c may yield erroneous results due to no-analog problems, we attempted to use a revised method (Mix et al., 1999) to generate a transfer function for extracting paleo-SSTsignals from planktic foraminifer abundance data. The revised method consists of two steps. First, the procedure involves de¢ning foraminifer Q-mode factor assemblages in MD972142 300 downcore samples, which re-sulted in two factors that represented 95% of the variance of the downcore faunal data. These two factors were : factor 1 (Globigerinoides ruber, Glo-bigerinoides sacculifer ; 51% of the variance) and factor 2 (Neogloboquadrina dutertrei+Neoglobo-quadrina pachyderma (right coiling) ; 44% of the variance). Next, the loadings of these two factors were mapped onto published western Paci¢c and SCS coretop data (Prell, 1985; Chen and Prell, 1998 ; Ho et al., 1998; P£aumann and Jian, 1999) and the coretop samples with factor com-munalities 6 0.6 were excluded. The remaining 459 coretop samples (communalities s 0.6) were then used as a calibration data set to generate transfer functions for estimating SSTs (Yu, 2000). The root mean square (RMS) errors of estimated cold (January)- and warm (July)-season SSTs in the equations are 2.3‡C and 1.4‡C respec-tively, somewhat larger than those in previous es-timation methods. The larger RMS errors shown in the revised transfer function method result from the fact that factor assemblages were de¢ned based on downcore samples and not speci¢cally tuned to ¢t the coretop samples (Mix et al., 1999). We felt that this revised method might improve the accuracy of our SSTestimates for the SCS records, as in Mix et al.’s (1999) experiments with equatorial Atlantic and eastern Paci¢c data sets, in which the discrepancy in estimating LGM tropical cooling between data and model appears to have been solved.

The 560 analyses for biogenic carbonate and organic carbon concentrations were conducted at National Taiwan Ocean University (Fig. 4). The samples were crushed to a ¢ne powder after dry-ing at 50‡C and split into several sub-samples for analysis. We measured the total carbon content (TC) of the samples with a HORIBA EMIA-8210 carbon analyzer. The procedure involves heating the samples to V1300‡C and measuring the combustion product CO2 gases by gas

chro-matography. The resulting precision was T 0.8% of the values being measured. The carbonate con-tent of the samples was determined by a fuming method. HCl was used to remove the carbonate content (TIC (total inorganic carbon)) of separate sub-samples. After the acid reaction, the sub-sam-ples were repeatedly analyzed for remaining car-bon content by the combustion method using the carbon analyzer. The carbonate content (TIC) could thus be calculated by subtracting total or-ganic carbon (TOC) from the TC values. In order to avoid forcing apparent anti-correlation in per-centage data, we calculated carbonate and TOC MARs (mg/cm2/kyr) (Fig. 4) based on the

follow-ing equation : MAR = SRU[BD3(PUWD)]U wt% (Sykes and Ramsay, 1995). In this equation, SR is the linear sedimentation rate, BD is the wet bulk density (g/cm3) measured from ship-board

MST(multi-sensor track) equipment, and WD is the seawater density ( = 1.025 g/cm3). P (%) is the

estimated porosity of carbonate or TOC sedi-ments. For calculating carbonate and TOC MARs, P was estimated with the following equa-tion : P = 0.72^0.045UZ (Simmons, 1990). Z (km) represents the depth of the core. When calculating sedimentation rate, and carbonate and TOC MARs, we found anomalously high values present in the upper part of the core down to a possibly maximum depth of about oxygen isotope stage 3 (Fig. 4). These anomalous MAR values may have resulted from a stretching of the upper part of the sediments collected with the giant pis-ton core CALYPSO on board the RV Marion Dufresne (Szeremeta et al., 2000), and were not interpreted to re£ect surface ocean variability in this study.

For spectral and cross-spectral analyses, the Mac user program ARAND (distributed by

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Brown University) was used. The spectra were calculated by the Blackman^Tukey method ( Je-kins and Watts, 1968) as used in SPECMAP stud-ies (Imbrie et al., 1992, 1993).

3. Results

The N18O record of the surface water dwelling

species Globigerinoides ruber follows closely the variations of the low-latitude N18O stack (Bassinot et al., 1994) (Fig. 2). The planktic foraminifer rec-ord documents six glacial^interglacial cycles, with large amplitudes varying from V2.0x (stage 1/ 2, 9/10, and 11/12 transitions) to 1.4x (stage 7/8 transition). The oldest part of the record included in this study is at the uppermost end of stage 13 ; it is estimated to date to V500 000 years ago. Sedimentation rates of this core are estimated to be V5 cm/kyr (Fig. 4), with no signi¢cant varia-tion between glacial and interglacial periods. These sedimentation rates are estimated here con-sistently with those reported from an eastern SCS core study (Thunell et al., 1992). The anomalous high values of sedimentation rates in the upper-most part of the core again could have been caused by stretching during the operation of the CALYPSO piston coring system.

The relative abundances of six dominant planktic foraminifer species are presented here in order to examine changes in sea-surface condi-tions, and climate changes in the southeastern SCS (Fig. 3). We observed large downcore varia-tions in the relative abundances of these dominant species, indicating that considerable change in the surface water has occurred in this region over the past 500 000 years. The species abundance pat-terns contain major low-frequency glacial^inter-glacial and also high-frequency variations. For example, the abundances of a warm-water, deep mixed-layer species Globigerinoides ruber in-creased noticeably during glacial (stage 4, 8, 12) and interglacial periods (stage 1, 7, 9, 11) and reached a maximum in late stage 11 and early stage 12. Although the abundances of a cold-water, high productivity/upwelling indicator spe-cies Neogloboquadrina dutertrei showed high val-ues during most glacial periods (stages 2, 4, 6, 8,

10, 12), there were also some peak values during interglacial periods (stages 3 and 7). The dances of the other species also showed abun-dance maxima in both glacial and interglacial stages, and £uctuated in a high-frequency mode. The faunal abundance patterns suggest an insta-bility in the surface ocean system of the SCS dur-ing the late Quaternary.

The planktic foraminifer SST estimates vary between 23.0‡C and 26.5‡C for the cold season (January) and between 27.4‡C and 29.2‡C for the warm season (July) over the past 500 000 years. The amplitudes of the SST changes (3.5‡C and 1.8‡C) observed in this record easily exceed the estimation errors (2.3‡C and 1.4‡C) in cold-and warm-season equations. The coretop values of the seasonal SSTs were V26.5^29‡C, in agree-ment with the approximate range V27^29‡C of modern SSTobservations at this site (NOAA, 1994). The estimates for LGM cooling at this site based on this record are V3‡C (January), which appears to be of the same magnitude as the cooling reported previously based on Uk

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re-cords (Huang et al., 1997a,b ; Pelejero et al., 1999a,b). However, they are smaller than those based on planktic foraminifer faunal estimates (V6‡C) (Chen and Huang, 1998 ; Chen et al., 1999) obtained by using a global transfer function (Ortiz and Mix, 1997). This suggests that the re-vised transfer function method (Mix et al., 1999) adopted in this study produces more consistent estimates with those reported from previous stud-ies, as the revised procedure better describes the total downcore faunal variation and more prop-erly relates the faunal variation to local oceano-graphic processes. The 3‡C magnitude of tropical LGM cooling that we estimated here also seems to be consistent with that reported in a recent re-evaluation (Schneider et al., 2000) and with that obtained through modeling (Hostetler and Clark, 2000).

The 500 000-year record generally exhibits a pattern of glacial decreases and interglacial in-creases in the faunal transfer function SSTesti-mates, but with several exceptions of large-ampli-tude cooling during interglacial periods and warming events within glacial periods. For exam-ple, several cooling events were observed in

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inter-glacial stages 5 and 7; and some warming epi-sodes occurred in glacial stages 6, 8, and 12 (Fig. 5). All of these episodes indicate that the surface ocean temperature variability in the SCS during the past 500 000 years was driven by more complicated factors than simply ice-volume forc-ing. Based on this record, we observed that large cooling spikes in this time series are near or close to the time intervals 10^60 ka, 140^150 ka, 250^ 260 ka, 340^360 ka, and 430 ka ; some extreme warming events lie close to the intervals 50 ka, 70^95 ka, 120 ka, 170 ka, 210 ka, 235 ka, 280 ka, 320 ka, and 370^420 ka. Superimposed on the high-frequency variations, the SSTrecords are characterized by more frequent and abrupt cooling events beginning V250 000 years ago. Large cooling events appear to be more evident after glacial stage 8 and become much more in-tensi¢ed in stage 2^4. Overall, the SSTrecords are not well-correlated with glacial and interglacial cycles.

Unlike SSTvariations, the carbonate concen-tration record of MD972142 exhibits signi¢cant variation at the broad glacial^interglacial cycles characteristic of the Globigerinoides ruber N18O

time series (Fig. 4). The carbonate content makes up V15^55 wt% of the MD972142 sediments, usually with maxima in interglacial and minima in glacial periods. The MD972142 carbonate rec-ord is consistent with that of a nearby core (GGC-9 (1465 m)) from the Palawan slope ( Thu-nell et al., 1992). Several spikes of low carbonate content are located near the intervals of tephra layers that are not sampled for the carbonate analyses. The downcore pattern of carbonate con-tent corresponds to previously reported SCS car-bonate sedimentation patterns from depths above the regional lysocline where terrigenous inputs are thought to be dominant (Thunell et al., 1992 ; Wang et al., 1995 ; Chen et al., 1997). In attempt-ing to minimize the problems of terrigenous dilu-tion and other biogenic components, we have

quanti¢ed the variability of carbonate £ux into the sediment by calculating carbonate MARs (Fig. 4). In the uppermost part of the record, the carbonate MARs show anomalously high val-ues which are presumably associated with the stretching of the core. From the depth at stage 3 to the bottom of the record, the carbonate MARs vary between V500 and 2000 mg/cm2/kyr, with

high values centered primarily in interglacial peri-ods (with an exception in stage 5) or in glacial^ interglacial transitions. The peaks in carbonate MARs during glacial^interglacial transitions are evident during stage 5^6, 7^8, and 9^10 transi-tions, and are most likely due to changing preser-vation. Many earlier observations (Peterson and Prell, 1985; Thunell et al., 1992 ; Chen et al., 1997) on Indian Ocean or SCS cores indicated a presence of carbonate preservation spikes during deglaciation stages. Given the close correspon-dence between our calculated carbonate MAR and content, the ¢rst-order variations of the car-bonate MARs are probably driven by glacial^in-terglacial productivity and/or carbonate preserva-tion changes in the SCS. As productivity in the SCS during interglacial stages is not likely high (Thunell et al., 1992) and the preservation level in the Indo-Paci¢c Ocean during interglacial stages is generally lower than that in glacial, the carbonate MAR and content variation patterns seem likely to indicate the changing terrigenous sediment supplies which are associated with gla-cial^interglacial sea-level £uctuations in the SCS. TOC concentrations constitute V0.2^1.4 wt% of the sediments, with maxima usually in glacial periods and minima typically in interglacial peri-ods (Fig. 4). The glacial^interglacial variations in TOC content have also been reported from near-by short gravity cores and have been used to dem-onstrate high glacial productivity in low-latitude surface oceans (Thunell et al., 1992). The general downcore pattern of the TOC maxima and mini-ma exhibits more low-frequency variability than

Fig. 4. Sedimentation rate, carbonate MAR, TOC MAR, and carbonate and TOC concentrations measured in sediment samples from core MD972142, plotted against age and compared to downcore N18O stratigraphy. Shaded intervals indicate glacial stages.

Anomalously high values shown in the sedimentation rate and MAR are caused by the stretching of the uppermost part of the core.

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that shown in the carbonate records. In addition to a maximum event centered on V430 ka, the record also shows a long-term trend with in-creased TOC content since 400 000 years ago. Over the broad, low-frequency curve of the TOC content variations, intervals with high val-ues are observed from V10^60 ka, 95^110 ka, 140^150 ka, 250^260 ka, 340^360 ka, and at

430 ka. These high TOC events seem to occur during the later stages of glacial periods. When converting the TOC content into TOC £ux (a proxy indicator for surface productivity), our cal-culations show that, in spite of the fact that the uppermost part of the record is a¡ected by stretching of the core, the TOC MARs (Fig. 4) vary between V5 and 30 mg/cm2/kyr and match

Fig. 5. Measured TOC concentrations, calculated TOC concentrations at constant productivity (147 g C/m2/yr; San

Diego-McGlone et al., 1999) and variable sedimentation rates (TOC expected, thin line), di¡erence between measured and calculated TOC concentrations (TOC residual = measured minus calculated TOC, dotted line), and SSTs for cold- and warm-season esti-mates by planktic foraminifer transfer functions from core MD972142, plotted against age and compared to downcore N18O

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well the variations of TOC content. Several high TOC intervals, for example, from V200 ka, 250^ 260 ka and 340^360 ka, become more pronounced in the MAR curve. Increasing sedimentation rates have also been thought to enhance TOC preser-vation (Mu«ller and Suess, 1979 ; Sarnthein et al.,

1988). When assessing past surface ocean produc-tivity changes, the pattern of TOC content seems to provide more reliable results than do TOC MARs. In subsequent analyses, our interpretation of the biogenic components of MD972142 will be primarily based on concentration records.

Fig. 6. Comparisons of MD972142 TOC, ODP 723 TOC (summer monsoon proxy) (Emeis et al., 1995) from the Arabian Sea, and Gulf of Aden benthic productivity and G. bulloides% (winter monsoon indicators) (Almogi-Labin et al., 2000) records.

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4. Discussion

The records of planktic foraminifer SSTs and TOC-based productivity indicators from core MD972142 indicate signi¢cant £uctuation during the past 500 000 years. Since the SCS is a margin-al sea of the western Paci¢c, the SST, surface circulation, and biogenic sedimentation in the SCS must be a¡ected by sea-level changes (Wang et al., 1999a,b). During times of lowered sea-level (glacials), the SCS SSTmay have de-creased because the surrounding continental shelves of the SCS emerged and restricted the in-£ow of warm surface waters from low-latitude oceans. Moreover, more down-slope or laterally advected sediment transports may occur during glacial periods due to the redistribution of sedi-ments in response to sea-level changes. When in-terpreting the MD972142 records in terms of past monsoon variability, it is necessary to understand how much of the variation exhibited in the re-cords is due to changes in sea-levels. As shown in the carbonate record (Fig. 4), glacial parts of the sediment core are characterized by relatively low carbonate content. The core site is presently located at a water depth of 1557 m, which is well above the regional lysocline (V3500 m) and also above an estimated glacial lysocline depth (V2500 m), based on abundance data of ptero-pods in gravity cores collected from a bathymetric transect along the Palawan slope (Thunell et al., 1992). Glacial^interglacial changes in carbonate preservation do not seem to be e¡ective in mod-ifying the carbonate content of the core. More-over, as carbonate preservation and productivity levels are both enhanced during glacial periods in the Paci¢c (Farrell and Prell, 1989 ; Sarnthein et al., 1988), all instances indicate that the glacial low carbonate content shown in core MD972142 was most likely driven by increased terrigenous sediment dilution e¡ects. Increased siliceous

com-ponent concentrations are also not possibly re-sponsible for the dilution e¡ects, since siliceous fossils (diatoms and radiolarians) are rare in these sediment samples. Since this site is located far away from the large rivers that drain into the SCS, transport of riverine sediments to this site by lateral advection may not be signi¢cant. We inferred that the increased terrigenous inputs are primarily caused by local redistribution of sedi-ments along the shelf-slope pro¢le of the Palawan. More terrigenous material may have been brought by various down-slope transfer processes, when sea-level fell during glacial periods. A sea-level reconstruction in the SCS based on changes in shoreline sedimentary facies appears to be parallel to global records (Hanebuth et al., 2000). Recent U37

k studies (Pelejero et al., 1999a,b ; Kienast et

al., 2001) using SCS cores also suggest a

synchronized relationship between SCS SSTand high-latitude climate change. By accepting the premise that sea-level e¡ects left their imprint on the MD972142 records, the ¢rst-order variations exhibited in the SSTand TOC and characterized in terms of glacial and interglacial cycles may be partially attributable to sea-level £uctuations in the SCS.

Atmospheric circulation changes may also play a role in controlling glacial SCS SSTand TOC. When we examined the SSTand TOC records together (Fig. 5), we found that the glacial periods were characterized by relatively low SSTs and high TOC content, indicating cooling and high productivity conditions in surface oceans. Pre-vious studies on SCS marine records (Chen and

Huang, 1998) and Chinese loess (Ding et al.,

1994, 1995) have already pointed out the impor-tance of ice-volume forcing in determining the intensity of East Asian winter monsoon winds, via a mechanism of latitudinal shifting position and/or strength of Siberian high-pressure systems. Large-scale glaciation in the continents of the

Fig. 7. Cross-spectral analysis between MD972142 estimated SSTfor cold- and warm-seasons, and the planktic foraminifer Globi-gerinoides ruber N18O for global ice-volume estimates. Notice that the N18O was multiplied by 31 to transform the interglacial

values into positive and glacial values into negative. Three major orbital frequency bands are shaded (100 kyr31, 41 kyr31, and

23 kyr31), in which statistically signi¢cant coherencies between these two time series and the phase relationships of SSTrelative

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Northern Hemisphere may intensify the high-pressure systems, which in turn bring more cold northeasterly winds to the SCS. The low SSTs and high productivity observed during the glacial periods of the MD972142 record may also re£ect strong wind-induced mixing, and nutrient enrich-ment with high productivity (Ittekkot et al., 1992).

Some rapid, high-frequency oscillations shown in the SSTand TOC records which do not closely parallel the N18O curve could be due to other

forc-ing mechanisms. Because high TOC content may result from increased sedimentation rates that en-hance TOC preservation, the in£uences of sedi-mentation rate changes on TOC content must be estimated. To this end, we calculated expected TOC content resulting under conditions of con-stant productivity and varied sedimentation rates by an equation : TOC expected (%) = (0.0030U RUS0:30)/p

s (Mu«ller and Suess, 1979) (Fig. 5). R

represents the observed regional productivity in the SCS (147 g C/m2/yr; San Diego-McGlone et al., 1999). S is the estimated sedimentation rate (cm/kyr) for the core, and ps is the estimated dry

bulk density for the core. This calculation yields changes in TOC content of less than V1%, esti-mated from varied sedimentation rate and con-stant productivity at the site of core MD972142. The large increases in expected TOC content in the uppermost part of the core are due to core stretching. Subtracting the expected TOC content of constant productivity from the measured TOC content yields a record of residual TOC content, which is attributable to changing productivity. The residual record preserves TOC content varia-tion of similar timing but rather damped ampli-tude. Examining the SSTand TOC together, we found several periods of relatively low SSTand high productivity at 10^60 ka, 95^110 ka, 140^ 150 ka, 250^260 ka, 340^360 ka, and at 430 ka.

These intervals seem to re£ect more intensi¢ed winter monsoon winds in the SCS. Even more interesting, these intervals correspond to events of stronger winter monsoons over the Arabian Sea, based on a record of the planktic foraminifer G. bulloides abundances and benthic foraminifer productivity indices in a core from the Gulf of Aden (Almogi-Labin et al., 2000) (Fig. 6). The Gulf of Aden is a semi-enclosed gulf, which re-ceives little in£uence from the summer monsoon upwelling prevailing in the Arabian Sea. During winter monsoon seasons, the surface waters in the Gulf of Aden experience cooling and mixing, with nutrient-rich subsurface waters injected into the photic zones. Therefore, the Gulf of Aden core records Indian Ocean winter monsoon variability (Almogi-Labin et al., 2000). Because the age mod-el of this core was constructed based on the SPECMAP time scale, correlation between events registered in both records is considered to be ac-curate. In addition, several intervals are charac-terized by relatively high SSTand low productiv-ity at 50 ka, 70^95 ka, 120 ka, 170 ka, 210 ka, 235 ka, 280 ka, 320 ka, and 370^420 ka. Calcu-lations of monthly averaged wind-stress ¢eld and divergence of Ekman transport (Hellerman and

Rosenstein, 1983) show downwelling and

deep-ened mixed-layer conditions present o¡ the coast of the northwestern Palawan during July, the summer monsoon season in the SCS. These inter-vals appear to indicate short episodes of intensi-¢ed summer monsoon winds in the SCS. Compar-ing our records to many previously published Indian Ocean summer monsoon reconstructions, we found that the variation patterns shown in these records have a similar structure in terms of timing of intensi¢ed summer monsoons. (Some are of relatively low resolution ; see : Clem-ens et al., 1991; Murray and Prell, 1992;

Ander-son and Prell, 1993.) The correspondences

be-Fig. 8. Cross-spectral analysis between MD972142 carbonate and TOC concentrations, and the planktic foraminifer Globigeri-noides ruber N18O for global ice-volume estimates. Notice that for carbonate spectra, the N18O was multiplied by 31 to transform

the interglacial values into positive and glacial values into negative. Three major orbital frequency bands are shaded (100 kyr31,

41 kyr31, and 23 kyr31), in which statistically signi¢cant coherencies between these two time series and the phase relationships of

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tween these two records are especially evident over the intervals 50 ka, 70^95 ka, 120 ka, and 210 ka. The strong summer monsoon intervals identi¢ed from the MD972142 record match very well, with a more high-resolution summer monsoon indicated by the U37

k and TOC record

from Arabian Sea ODP Site 723 (Emeis et al., 1995) (Fig. 6). By combining all the correlations found in the records from the SCS and the Ara-bian Sea, it is possible to infer that the MD972142 records contain a mix of in£uences from both winter and summer monsoons. The coincidence of intensi¢ed winter and summer monsoon events expressed in both the SCS and Arabian Sea data indicates more regionally e¡ective patterns of the Asian monsoon system. This comparison also suggests that there are coherent patterns of tem-poral changes in di¡erent parts of tropical mon-soon-a¡ected areas over glacial^interglacial time scales.

We performed spectral and cross-spectral anal-yses to evaluate the dominant periodicities appar-ent in the MD972142 records. We also looked at the coherent and phase relationships between the records and global climate change over orbital time scales (104^105 years). In the variance density

spectra of the SSTs (Fig. 7), statistically signi¢-cant variance peaks occur at all orbital frequen-cy bands (100 kyr31, 41 kyr31, and 23 kyr31).

The presence of a 100-kyr periodicity in the MD972142 SSTrecords suggests an ice-volume-and/or sea-level-related forcing on SCS monsoon changes. The dominances of 41 kyr and 23 kyr in the SSTrecords imply an orbitally driven solar insolation in£uence on the SCS monsoon systems, as previously proposed in Indian Ocean monsoon studies (Clemens et al., 1991). The phase relation-ships between the SSTand Globigerinoides ruber N18O (ice-volume indicator) records from the same core analyzed by cross-spectral analyses (Fig. 7) reveal consistent scenarios. Signi¢cant coherencies are observed at 100-kyr31 and 23-kyr31frequency

bands. Maximum MD972142 SSTs are nearly in-phase with minimum ice volumes over 100-kyr cycles, indicating a response to glacial boundary conditions (Anderson and Prell, 1993 ; Ding et al.,

1995 ; Chen and Huang, 1998). The maximum

SSTs lag minimum ice volume by V3000^4000

years at 23-kyr cycles. Others have reported the same phase lagging of Indian Ocean summer monsoon intensities in this cycle, although at dif-ferent magnitudes of phase lags (Clemens et al., 1991).

For carbonate content records, spectral analy-ses (Fig. 8a) indicated statistically signi¢cant var-iance peaks in all three orbital frequency bands. Using cross-spectral analysis, we found that the carbonate record is strikingly coherent and in-phase with ice-volume changes at 100-kyr cycles. Maximum carbonate contents are associated with minimum ice volume and/or high sea-level condi-tions. The peaks at 41-kyr31 and 23-kyr31

fre-quency bands shown in the carbonate record are indications of a combination of in£uences from sea-level, carbonate productivity and dissolution, or even more complicated factors, since no signi¢-cant coherent and in-phase relationship is ob-served at these two frequency bands. The spec-trum of the MD972142 TOC content (Fig. 8b) exhibits orbital and non-orbital peaks. At orbital frequency bands, the TOC variation shows a no-ticeable peak in the 100-kyr cycle. Maximum TOC contents are coherent and nearly in-phase with maximum ice volume at this frequency band. This suggests that the production and/or preservation of MD972142 TOC is primarily linked to sea-level or ice volume-induced winter monsoon wind e¡ects. The non-orbital cyclicities (V70 kyr and 35 kyr) evident in the TOC record are also dominant in Indian Ocean summer mon-soon records (Clemens and Prell, 1991) and equa-torial Paci¢c eolian grain size and radiolarian di-vergence records (Pisias and Rea, 1988). The climatic linkages among the non-orbital cyclicities shown in these records, although requiring further investigation, suggest that more complex forcing mechanisms than previously thought may govern tropical climate variability over orbital time scales.

5. Conclusions

This study of planktic foraminifer faunal SST estimates and biogenic carbonate and TOC con-centrations from an IMAGES core record from

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the southeastern SCS near Palawan Island reveals that SSTs and TOC are good proxies for East Asian monsoon strength. The carbonate content record is dominated by terrigenous sediment in-puts, indicating an environmental in£uence from sea-level £uctuations in this record. The record shows that glacial periods are usually character-ized by relatively low SSTs and high TOC con-tent, while interglacial periods are characterized by relatively high SSTs and low TOC content. This association provides an indication that the temperature and productivity levels in the surface ocean of the SCS are largely controlled by sea-level £uctuations and/or ice volume-induced at-mospheric circulation changes in monsoon wind intensities in the SCS. Several short-lived, high-frequency oscillation intervals shown in SSTand TOC records which are not parallel to the N18O

curve are attributed to other forcing factors that control the monsoons. These intervals of intensi-¢ed winter and summer monsoons can be traced beyond the SCS. When we compared the MD972142 record with Arabian Sea winter and summer monsoon variability reconstructions, we found a remarkably close correspondence. More-over, the records from these two regions share a similar structure in terms of the timing of intensi-¢ed monsoon winds. We infer that the MD972142 records contain combined in£uences from both winter and summer monsoons. The coincidence of intensi¢ed winter and summer monsoon events expressed in both the SCS and Arabian Sea indi-cates more regionally e¡ective patterns of the Asian monsoon systems. The monsoon indices show strong peaks at three orbital frequency bands (100 kyr31, 41 kyr31, and 23 kyr31),

sug-gesting that over orbital time scales, ice volume and orbital solar insolation changes are all possi-ble mechanisms for controlling SCS monsoon var-iations. The SCS SST lags ice volume by V3000^ 4000 years at 23-kyr cycles, consistent with that reported from Arabian Sea summer monsoon re-cords. Although this should also be tested by oth-er proxies, our ¢nding suggests a more regionally coherent temporal pattern of monsoon variations in the Indian Ocean and the SCS than previously assumed.

Acknowledgements

This research was supported by the National Science Council (NSC89-2611-M-019-032-IM), Academia Sinica (Asian Paleoenvironmental Changes (APEC) Projects), and National Taiwan Ocean University, Republic of China. We thank Hodaka Kawahata, Nick Pisias, and Franck Bas-sinot for their constructive reviews.

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數據

Fig. 1. Map showing the position of the IMAGES core MD972142.
Fig. 2. The oxygen isotope (Globigerinoides ruber) stratigraphies of the IMAGES core MD972142 which is estimated to represent early stage 13 through the Holocene
Fig. 3. Relative abundances of six dominant species of planktic foraminifera for core MD972142, plotted against age and com- com-pared to downcore N 18 O stratigraphy
Fig. 5. Measured TOC concentrations, calculated TOC concentrations at constant productivity (147 g C/m 2 /yr; San Diego- Diego-McGlone et al., 1999) and variable sedimentation rates (TOC expected, thin line), di¡erence between measured and calculated TOC c
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